Directly measured tissue pH is an earlier indicator of splanchnic acidosis than tonometric parameters during hemorrhagic shock in swine Juan Carlos Puyana, MD; Babs R. Soller, PhD; Bhairavi Parikh, MS; Stephen O. Heard, MD, FCCM G astric and intestinal pH (pH i ) and PCO 2 derived by tonome- try have been shown to be indicators of hypoperfusion in animal models of endotoxemia, mes- enteric occlusion, hemorrhagic shock, and cardiac tamponade (1– 6). The tech- nique has been applied clinically because it is the only available minimally invasive measure of regional hypoperfusion and ischemia. Gastric PCO 2 and its derivatives, pH i and PCO 2 gap (gastric luminal CO 2 - arterial CO 2 ), have been shown to be use- ful in predicting outcome of critically ill patients, monitoring the effectiveness of resuscitation, and determining multiple organ dysfunction (7–14). However, there is still considerable controversy sur- rounding the best indicator for assess- ment of splanchnic hypoperfusion. Sev- eral authors have discussed the limitations of calculating pH i from arte- rial bicarbonate values and have proposed directly measured PCO 2 or the gap be- tween tissue and arterial PCO 2 as the best indicator (15–20). Tonometry directly measures the par- tial pressure of CO 2 in the lumen of the stomach or the intestines. Luminal PCO 2 is believed to be in equilibrium with tis- sue PCO 2 and therefore may be an indica- tor of cellular metabolic status. When oxygen delivery is significantly reduced, increases in tissue PCO 2 result primarily from the buffering of hydrogen ions pro- duced when there is an imbalance be- tween protons released by adenosine triphosphate hydrolysis and consumed by adenosine triphosphate synthesis. (21, 22). Ideally, a direct measurement of tis- sue pH would provide the desired meta- bolic assessment and would be uncompli- cated by other factors that can affect luminal PCO 2 , such as changes in sys- temic PCO 2 and bicarbonate produced from the duodenum, stomach, and pan- creas. Recently, a minimally invasive method for direct measurement of tissue pH using near-infrared spectroscopy has been demonstrated in skeletal muscle and bowel (23–25). This minimally inva- sive technique could ultimately provide the same clinical ease as tonometry. In this investigation, we compared tissue pH in the stomach, bowel, and abdominal wall muscle during hemorrhagic shock using a direct measurement of pH with From the Department of Surgery, Brigham & Wom- en’s Hospital, Boston, MA (Dr. Puyana), and the De- partments of Surgery (Dr. Soller, Ms. Parikh) and An- esthesiology (Dr. Heard), University of Massachusetts Medical School, Worcester, MA. Presented, in part, at 27 th SCCM Scientific Sym- posium, Feb. 5, 1998, San Antonio, TX. Supported, in part, by the University of Massachu- setts/Smith & Nephew Center for Research in Endo- scopic Surgery and the U.S. Army Medical Research Command. The views, opinions, and/or findings con- tained in this report are those of the authors and should not be construed as an official Department of Army position, policy, or decision unless so designated by other documentation. Copyright © 2000 by Lippincott Williams & Wilkins Objective: To compare tissue pH in the stomach, bowel, and abdominal wall muscle during hemorrhagic shock and recovery using tissue electrodes; also, to compare tissue electrode pH measurements to gastric intramucosal pH (pH i ), gastric luminal PCO 2 , and PCO 2 gap (gastric luminal CO 2  arterial CO 2 ) measured with an air-equilibrated tonometer. Design: Prospective animal study. Setting: University animal research laboratory. Subjects: Eight anesthetized, mechanically ventilated York- shire swine. Interventions: Hemorrhagic shock was initiated by withdraw- ing blood over a 15-min period to lower systolic blood pressure to 45 mm Hg. Shock was maintained for 45 mins and was followed by a 5-min resuscitation to normal blood pressure with a blood/ lactated Ringer’s (1:2) mixture. Recovery was monitored for 60 mins. Measurements and Main Results: pH was measured with elec- trodes in the submucosa of the stomach, the submucosa of the small bowel, and the abdominal wall muscle. Gastric luminal PCO 2 was measured with an air-equilibrated tonometer and pH i and PCO 2 gap were calculated. Each organ showed a different sensi- tivity to shock and resuscitation. The bowel pH responded most rapidly to the onset of hemorrhagic shock and had the largest change in tissue pH. The bowel also showed the most rapid recovery during resuscitation. The submucosal pH of the stomach responded more slowly than the bowel, but faster than the ab- dominal wall muscle pH, gastric PCO 2 gap, or pH i . The smallest changes in organ pH as a result of hemorrhagic shock were seen in the abdominal wall muscle and the stomach as assessed by gastric tonometry. Conclusions: Direct measurement of tissue pH indicates that intra-abdominal organ pH varies during hemorrhagic shock. The small bowel pH changes the most in magnitude and rapidity compared with stomach pH or abdominal wall muscle pH. Tono- metrically derived parameters were not as sensitive in the detec- tion of tissue acidosis during shock and resuscitation as pH measured directly in the submucosa of the stomach or small bowel. (Crit Care Med 2000; 28:2557–2562) KEY WORDS: hemorrhage; shock; tissue pH; small bowel; gastric tonometry; monitoring 2557 Crit Care Med 2000 Vol. 28, No. 7